Fabricating methods used to produce molded reinforced polyester articles range from simple open mold fabrication, known as contact molding, to the matched-die method, one of the most advanced in terms of mechanization. The contact molding process is probably the oldest method of producing such reinforced structures and is still widely used where it is desirable to keep tooling costs to a minimum.
The contact molding process involves the use of a single mold conforming either to the outside or inside dimensions of the finished part. Usually the mold is first coated with a release agent to assure release of the molded part. Often a "gel-coat" of pigmented resin is applied to the surface of the mold and allowed to set. A reinforcing glass fiber mat, pre-formed to conform with the shape of a mold is placed within the mold and a pre-determined quantity of liquid polyester resin and chopped glass fibers is poured over the mat. The polyester resin-glass fibers can also contain a catalyst, associated fillers, pigments, etc. The resin-glass fiber mixture is then worked into the reinforcing mat.
To insure formation of a uniform product, the reinforcing mat must be completely wetted by the resin-glass fiber mixture. Whether the resin is applied by spraying, brushing or rolling, a considerable amount of hand-work is required to work the resin into the mat in a uniform manner. This hand-work operation is a source of problems; particular care must be taken to avoid entrapment of air bubbles in the reinforcing mat and polyester resin, and non-uniformity of the glass fiber-resin composition. Improper hand-work can cause an uneven distribution of the resin and chopped glass fibers wherein certain areas of the final product have a build-up of resin at the expense of the reinforcing fibers, resulting in a product having decreased structural strength.
After each layer of reinforcing material is put into position, additional resin and working are required. Because of the need to increase the speed of the operation, rolling of the resin mixture into the reinforcing fiber mat is desirable over brushing. However, if the rolling is done by a roller having a conventional configuration, wherein the outer surface of the roller is in contact with the reinforcing mat, as the roller is moved across the mold a build-up of excess resin occurs on the front part of the roller. As the roller continues in its movement across the mold, the build-up of the excess resin increases until the reinforcing mat is lifted off the mold surface. When this occurs, the reinforcing mat is torn and displaced, producing what is referred to in the molding business as "wash-out". These problems, associated with contact molding techniques, can be avoided of course by matched-die molding which generally involves the use of two dies -- the male plug or force, and a female or cavity. Although the matcheddie process permits the use of fast production cycles with uniform construction, the high cost of the dies which are usually steel, cast iron or aluminum, and preferably chromeplated steel, is such that low production costs can only be achieved in the production of articles which are made in very large numbers. Consequently, there is a need for improvement of the contact molding process wherein the hand lay-up step can be conducted with increased speed, avoiding bubbles and "wash-out".